Cosine Flicker Photometer. 651 



When comparing lights of the same colour the sensitiveness 

 of the photometer seemed to be very much the same for both 

 methods, but when the comparison of sources of light differing 

 widely in colour was attempted, the Flicker method gave rise 

 to the most consistent results. 



It seems possible that the sensitiveness of a photometer of 

 this type might be improved by introducing the element of 

 contrast in the manner shown in fig. 6 (PL XVI.) 



Each of the photometrical surfaces is shown divided into 

 3 equal and similar portions. The portions 2 and 5 are 

 intended to be as white as possible. The portions 1 and 6 

 are tinted with a neutral wash of such depth as to reduce the 

 coefficient of reflexion from the surface by say 4 per cent., 

 while 3 and 4 are intended to be intermediate in shade 

 between 1 and 5 and 2 and 6 respectively. 



Now if the Equality of Brightness method is used, the 

 observer attempts to set the wedge so that 3 and 4 appear 

 equally bright. The contrast in light and shade exhibited 

 between 1 and 2 and 6 and 5 respectively should then be 

 the same. 



If, now, the wedge is rotated slightly in a clockwise 

 direction, the contrast between 1 and 2 becomes more marked 

 but that between 6 and 5 less marked than before. 



We have thus a double change such as occurs in the 

 Lummer-Brodhun contrast photometer and a corresponding 

 gain in sensitiveness. If the Flicker method is used a corre- 

 sponding effect will exist. 



The author has endeavoured to facilitate the comparison 

 of heterochromatic sources of light by applying Crova's 

 method. 



As the temperature of an incandescent solid is raised, the 

 luminous radiation in the form of waves of short wave-length 

 increases more rapidly than the luminous radiation of great 

 wave-length. 



According to the principle discovered by Grova*, however, 

 the luminosity in the spectrum of such a source in the neigh- 

 bourhood of X= 0*582^ is a measure of the total luminosity 

 of the source. Crova therefore proposed to view the surfaces 

 illuminated by two heterochromatic sources through a 

 solution allowing only rays in this portion of the spectrum 

 to pass. This solution was composed of : — 



Anhydrous ferric chloride, 22*32 gms. 

 Crystallized nickel chloride, 27*19 gms. 

 Distilled water ? 100 cc. 



* Comptes Rendus, vol. cxix. no. 16, p. 627. 



